<!DOCTYPE html>
<html lang="en">
<head>
    <style>
        html {
            font-size: 100% /* or 62.5% or whatever you like, it doesn't matter, it's just a browser fix, however "rem" units will be based on that value, so make it confortable for calculations */
        }

        body {
            font-size: 0.9em; /* That is your text's default font size. Here i chose 12px */
            background-color: #BACKGROUND#;
        }
    </style>
</head>
<body oncontextmenu="return false;">

<div id="box1" style="margin: 0 15px 0 0">
    <p>
        #TEXT#
    </p>
</div>
</body>
</html>